Zirconia (ZrO2) has three crystallographic forms. Its naturally occurring crystalline form is monoclinic zirconia, which is stable at 1 atmosphere pressure up to temperatures of about 1170° C. Between about 1170° C. and about 2370° C., the stable phase is tetragonal zirconia. Above about 2370° C., the stable phase is cubic zirconia. The different phases of zirconia can be identified by techniques well known in the art, for example by X-ray diffraction. For example, U.S. Pat. No. 4,316,964 describes how 2θ scans between 27 and 33° can be used to determine the ratio of tetragonal to monoclinic zirconia phases and scans between 55° and 62° can be used to determine the tetragonal or cubic zirconia structure.
The different phases of crystalline zirconia may be stabilized by adding certain stabilizing elements to the zirconia. For example, U.S. Pat. No. 4,316,964 describes how the tetragonal and cubic phases of zirconia may be provided in a meta-stable form at room temperature when the zirconia is doped with a dopant. The most common stabilizing elements include magnesium (Mg), calcium (Ca) and rare earth elements such as cerium (Ce), yttrium (Y), Erbium (Er), Ytterbium (Yb), dysprosium (Dy), titanium (Ti) and Hafnium (Hf).
Introducing these stabilizing elements into zirconia may be achieved by heating it with, for example, an oxide of a stabilizing element. Thus, for example, CeO2, Y2O3, Ca2O3, Er2O3, Yb2O3.Dy2O3, TiO2, HfO2, MgO and CaO may be added to the zirconia. Typically, stabilization in this way results in the formation of a solid solution of the stabilizing element(s) in the zirconia.
The different phases of crystalline zirconia are known to exhibit different properties. For example, tetragonal zirconia is known to exhibit high toughness. One explanation for this toughness of tetragonal zirconia is that, where a crack forms, the zirconia at the crack tip undergoes a phase transformation from tetragonal zirconia to monoclinic zirconia. This phase transformation is accompanied by an increase in volume of typically about 3 to 5%. This increase in volume induces a compressive stress that, in turn, acts to reduce the driving force for crack propagation. This mechanism is termed “transformation toughening” and is described in WO 90/11980.
It is also known that stabilized cubic zirconia may exhibit a similar effect when subject to cracking.
While zirconia may exhibit advantageous toughness properties by itself, it does not have ideal hardness for some applications. In view of this lack of hardness, alumina has sometimes been added to zirconia materials. When alumina and zirconia are mixed and heated, the alumina generally remains separate from the zirconia and mostly does not form a solid solution with the zirconia. An example of this approach is taken in EP 1217235.
In addition, the mechanism contributing to the toughness of zirconia is also thought to contribute to stabilized zirconia's poor retention of its mechanical properties at increasing temperatures. In particular, the stability of the tetragonal and cubic phases of zirconia increases with increasing temperature and, as a result, the tendency for phase transformation to occur at the tip of a forming crack decreases with increasing temperature. This poor retention of thermal characteristics is also addressed by the addition of alumina because, while alumina has lower strength and toughness than zirconia at ambient temperature, it retains its strength and toughness at increasing temperatures to a greater degree than zirconia. In addition, alumina has a higher thermal conductivity and lower thermal expansion than zirconia, which helps prevent thermal shock.
Separately, WO 90/11980 describes how strontium, for example provided as an oxide of strontium such as strontium oxide (SrO), does not dissolve in an alumina/zirconia mixture but instead forms a separate strontium aluminate phase. This separate phase is a discontinuous phase formed from “platelets” of strontium aluminate. The aluminate is thought to be present as SrO.6Al2O3.